Photoelectric effect, photons Physics 123 11/20/2018 Lecture X

Concepts Photoelectric effect Work function and stopping potential ElectronVolt (eV) Photons Blackbody radiation 11/20/2018 Lecture X

Photoelectric effect Light shines on metal surface P Electrons are emitted from P Current flows between P and C Measure kinetic energy of emitted electrons KEe by applying a stopping potential V0: KEe=eV0 No surprises so far, because light as a wave carries energy 11/20/2018 Lecture X

Electronvolt Energy that one electron gains when being accelerated over 1V potential difference is called electronvolt eV: 1eV=1.6x10-19C 1V= 1.6x10-19J Yet another unit to measure energy, Commonly used in atomic and particle physics. 11/20/2018 Lecture X

Photoelectric effect Inside metal electrons are sitting in potential “wells” Need supply some minimum energy W0 to get them out W0 is called work function – different for different materials Light provides this energy EL EL=Ne(W0+KEe) Ne- number of emitted electrons e- W0 light 11/20/2018 Lecture X

Photoelectric effect EL=Ne(W0+KEe) Here is a surprise: Increase light intensity EL– expect More electrons Ne - True With higher kinetic energy KEe – False Do not expect KE to depend on light wavelength l , but it does More over if l>l0 – no electrons come out no matter how intensive the light source is!!! 11/20/2018 Lecture X

Photoelectric effect W0 Explanation of photoeffect A.Einstein Postulate: light is transmitted in tiny particles (!!) – photons (g) Each photon carries energy proportional to its frequency: Eg=hf=hc/l Planck’s constant h=6.626 x 10-34 J.s hc=1243 eV.nm One electron absorbs only one photon If Eg<W0 – no electrons are emitted: Eg<W0 f<f0 l>l0 Eg Eg Eg e- W0 f0=W0/h l0=hc/W0 11/20/2018 Lecture X

Photoelectric effect Eg=hf=hc/l Photon energy (Eg) is spent to get electron out (W0) and on electron’s kinetic energy (KEe): Eg= W0+ KEe If stopping potential is applied electron’s kinetic energy KEe is converted into potential energy of the electron eV0: Eg= W0+ eV0 Eg Eg Eg KEe e- W0 11/20/2018 Lecture X

Photons – particles of light Photon= particle of light = quantum of light = gamma(g)-quant Intensity of electromagnetic wave – sum of energies carried by quanta of light – photons: I=NEg Each photon carries energy proportional to the frequency of the EM wave: Eg=hf=hc/l 11/20/2018 Lecture X

Problem 38-15 I=0 when l>l0=570nm Work function W0-? What stopping potential (V0) must be applied if light of l=400 nm is used? 11/20/2018 Lecture X

Photon absorption and emission Photons can be absorbed by matter – converted into other forms of energy – e.g. kinetic, potential, thermal. At this point photon ceases to exist Final energy=Initial energy+Eg Photons can be emitted thus reducing the energy of the remaining system. Photon was NOT hiding inside matter waiting to be released, it is created by converting other forms of energy into light (EM-wave) Final energy+Eg=Initial energy Energy is conserved in either case 11/20/2018 Lecture X

More particle properties of light Sun tan – chemical reaction with threshold energy W0 visible light l=700-400 nm does not have enough energy to start this reaction ultraviolet – l<400 nm - more energetic light – does have enough energy Photographic film exposure Why red light is safe? Eg Eg Eg KEe e- W0 11/20/2018 Lecture X

Blackbody radiation Another problem was elegantly solved by introducing “light particles” Classical explanation of Blackbody (no reflection radiation diverged at low wavelength Max Planck suggested replacing integral over frequencies with summing a series (photons!) peak lpT=2.9x10-3 mK Planck did not seek deep meaning behind this seemingly mathematical trick Fundamental explanation was suggested by Einstein in 1905 – energy is quantized 11/20/2018 Lecture X